Apparatus for reducing finely divided iron oxide material

ABSTRACT

Apparatus for reducing finely divided iron oxide material, comprising a reactor containing a vertical upper reaction chamber connected downwardly to a narrower, vertical reaction chamber. A cyclone separator is connected to the upper reaction chamber for separating solid material and recycling it to the reactor so that a circulating fluidized bed can be maintained in the apparatus. In accordance with the invention, a recycling conduit is connected to the bottom of the lower reaction chamber. A tapping-off shaft for reduced material is also connected to the bottom of the lower reaction chamber. A reducing agent is supplied to the upper reaction chamber, and combustion air is supplied to the bottom of the upper reaction chamber. The apparatus also comprises means for preheating the iron oxide material with the exhaust gas from the reactor and for passing said preheated iron oxide into the lower reaction chamber. The apparatus also comprises means for stripping the exhaust gas from CO 2  and H 2  O and recycling it to the reactor to be used as fluidizing gas.

TECHNICAL FIELD

The present invention relates to an apparatus for completely orpartially reducing finely divided iron oxide material in a circulatingfluidized bed. The reduction should take place at a temperature lowerthan the melting point of the iron, and will take place in a mixture ofiron oxide material and solid carbonaceous material. The solidcarbonaceous material is preferably coke obtained from fuels andreducing agents containing carbon, e.g., anthracite, coal or oil. By"finely divided iron oxide-containing material" is intended iron oreconcentrates, roasted pyrites or other finely divided iron oxidematerials.

BACKGROUND ART

For the description of different fluidized beds and especiallycirculating fluidized beds, reference is made to L. Reh "Fluidized BedProcessing" Chemical Engineering Progress, Vol. 67, No. 2, February1971, pages 58-63.

Reference can also be made to the Swedish Patent specification 355 956,relating to a method for carrying out endothermic processes, whereinsolid material is treated in a reaction zone consisting of a heavilyexpanding fluidized bed, i.e. a circulating fluidized bed. A majorportion of the solid material is discharged together with gas in a upperpart of the fluidized bed reactor. A portion of the heat quantityrequired for the endothermic process is supplied to the fluidized bed bya gas having a temperature of at least 300° C. The solid materialdischarged at the upper part of the reactor is separated from the gas ina cyclone separator and is at least partially refluxed to the fluidizedbed. This known method is useful, i.a. in reduction processes, e.g. thereduction of gypsum, and chemical processes at high temperature, e.g.the oxidation of ilmenite.

It has already been proposed to produce a molten carbon-containing metalfrom a metal oxide, e.g. raw iron from iron oxide, by pre-reduction ofthe metal oxide followed by a smelting reduction, the heat requirementof which is provided to a major extent by electrical heating (SwedishPatent No. 7305753-1, publication number 393 816). An additional fuel,intended for the production of electrical energy, is used here forpre-heating, pre-reduction and other pre-treatment of the raw materialsfor the smelting reduction, and the remaining heat content in this fuel,as well as in the exhaust gases from the smelting reduction, are usedfor generating electrical energy which is utilized for the smeltingreduction.

The pre-reduction process as described above, i.e. reduction of finelydivided iron oxide material mixed with finely-divided solid carbonaceousmaterial, has been further developed in the way described in the SwedishPatent specification No. 7403145-1 (publication number 384 225), acirculating fluidized bed being utilized and a degree of reduction up to85% being obtained. In this known method the circulating fluidized bedis maintained in a vertically elongate reaction zone by adding to saidreaction zone adjusted flows of finely divided solid materials, andpossibly also a liquid carbonaceous material, and a gas containingmolecular oxygen, and by subjecting departing gas and solid materialfrom the reaction zone to a separation process, separated solid materialbeing recycled into the reaction zone. The finely devided iron oxidematerial, the carbonaceous material and the gas containing the molecularoxygen are supplied to an intermediate portion of the reaction zone, andthe flow of carbon supplied is controlled so that a proportion of cokeis always maintained in the bed such that disturbance of thefluidization due to stickiness is prevented. Gas and solid materialdeparting from the reaction zone are taken out from its upper part, theseparated solid material being recycled to the intermediate portion ofthe reaction zone. Solid material containing wholly or partially reducediron oxide is discharged from the lower part of the reactor zone.

The latter mode of operation has signified a clear improvement ofpre-reduction possibilities but it has been found that the result hasnot been entirely satisfactory. Difficulties have occurred, i.a. due tothe iron oxide particles and carbon particles segregating so that thelower part of the reactor is given a considerably greater content ofiron oxide than the upper part, while the situation is the opposite forthe carbonaceous material. This segregation has resulted in adeteriorated reduction process and in functional disturbances in theplant. The material has furthermore adhered to the air supply nozzlesand to the furnace wall around said nozzles.

DISCLOSURE OF THE INVENTION

The present invention provides a solution of these problems and relatesto an apparatus in which a product with a degree of metallization ofover 60%, and with a desired carbon content, can be produced withoutdisturbances due to segregation or sticking by the material. Theinvention relates to an apparatus for the complete or partial reductionof finely divided iron oxide material in a circulating fluidized bed,the apparatus including a preferably cylindrical, vertical reactorcontaining a vertical, elongate upper reaction chamber, a lower,narrower similarly vertical reaction chamber, a conical zone between thereaction chambers, a cyclone separator connected to the upper reactionchamber for separating the solid material from the fluidized bed andrecycling it via a recycling conduit to the reactor; supply means for areduction agent and for iron oxide material, nozzles for supplyingcombustion air to the reactor, means for supplying a reduction agent andan iron oxide material, means for tapping off reduced material, andmeans for supplying fluidized gas to the lower reaction chamber. Theapparatus of the invention is characterized in that the recyclingconduit for the solid material opens in the bottom of the lower reactionchamber. The sticking problem mentioned is eliminated to a considerabledegree in such an apparatus, and a high degree of operationalreliability can be obtained. The supply nozzles for the combustion airpreferably open in the conical zone or in the upper reaction chamber ata height above the conical zone of at most twice the diameter of thelower reaction chamber. The air nozzles preferably extend interiorly ofthe interior wall of the reactor, preferably a distance which is atleast equal to the inner diameter of the nozzles. The interior wall ofthe reactor preferably consists of a refractory lining.

It is preferred that the quantity of circulating solid material, i.e.the solid material separated in the cyclone separator and recycled tothe reactor, is higher than 10 times the quantity of fresh solidmaterial supplied to the apparatus.

DESCRIPTION OF THE DRAWINGS

Two embodiments of the invention are exemplified in the FIGS. 1 and 2.

According to the figures, an apparatus in accordance with the inventioncomprises a refractory-lined reactor 1 defining a vertically elongateupper reaction chamber 2, preferably cylindrical, and a lower reactionchamber 3, also preferably cylindrical. The height of the lower reactionchamber is 1/4-1/6 of the total height of the upper reaction chamber,and it has a cross-sectional area which is only 1/3-1/2 of that of theupper reaction chamber. Between the upper and the lower reaction chamberthere is a conical zone 4. Combustion air is supplied from a compressor5 through nozzles 6, which either open into the conical zone 4 (FIG. 2)or into the upper reaction chamber 2, at a distance from the conicalzone 4 of at most twice the diameter of the lower reaction chamber 3(FIG. 1). The nozzles extend from the inside face of the wall of thereactor and inwards thereof, and open out at a distance therefrom, whichis at least equal to the inside diameter of the nozzles. The diameter ofthe nozzles must be sufficient for the combustion air blown in to begiven an impulse or a momentum sufficient for rapidly mixing the airwith the content in the reaction chamber. The direction of the nozzlescan possibly deviate from the horizontal plane by at most 45° downwardsor at most 80° upwards. Above the nozzles there are one or more deliveryopenings for carbonaceous powder supplied through a conduit 7. The upperreaction chamber 2 is connected to a refractory-lined main cycloneseparator 8 for separating solid material in the departing gas. Aconduit 9 goes from the cyclone separator 8 to recycle the solidmaterial to the bottom of the lower reaction chamber 3. For the supplyof reducing fluidizing gas there are nozzles 10 opening out in thebottom of the lower reaction chamber 3. A narrow tapping shaft 11 isalso connected to the bottom, the shaft being downwardly connected via avalve 12 (FIG. 1) or fluidizing trap 13 (FIG. 2) with a cooling means 14for cooling the tapped material. A branch pipe 15 extends from the mainfluidizing gas conduit 32 to the lower portion of the tapping shaft 11.The gas thus flowing upwards through the shaft 11 prevents thedischarged material from packing together in said shaft, and may alsocreate a wind-separating effect in said shaft, so as to remove lightercarbon particles from the discharged material in the shaft 11. Gas isalso taken via a pipe 16 to the valve 12 as well, (FIG. 1), this valvebeing suitably powder-tight but not gas-tight, which prevents the powderfrom packing and clogging the valve.

The exhaust gas from the cyclone separator 8 is taken to a venturipreheater 17, in which it meets the ore concentrates which is fed in viaa supply pipe 18 (FIG. 1). Two or more dust cyclone separators 19, 20are connected in series to the venturi pre-heater 17, these cycloneseparators separating the ore concentrates from the departing gas andreturning the dust to the lower reaction chamber 3 via dust pipes 23, 24provided with gas traps 21, 22.

According to FIG. 2, the ore concentrates is preheated by the gas fromthe first dust cyclone separator 19 going to a second venturi preheater25, to which the ore concentrates is fed through a supply pipe 18. Theore concentrate is separated from the gas in a second dust cycloneseparator 26, and is discharged to the previously mentioned venturipreheater 17 via a gas trap 27. The exhaust gas is cleaned in anadditional dust cyclone separator 28. The solid material from the maincyclone separator 8 suitably passes through a gas trap 29. The gas traps21, 22, 27, 29, which can be sluice valves or powder locks, let dustthrough but prevent gas from flowing up into the cyclone separators. Tothe outlet 30 from the last dust cyclone separators 20, 28 there isconnected an outlet 31 for excess gas, suitably connected to an energyrecovery equipment, e.g. a steam power electrical plant. The outletconduit 30 is also connected to a conduit 32 which contains a gascompressor 34 and a washing apparatus 33 for removing H₂ O and CO₂ fromthe gas. The gas is conveyed to the bottom of the lower reaction chamber3 and discharge shaft 11.

FIG. 2 illustrates how the exhaust gas from the last dust cycloneseparator 28 is used in a heat exchanger 35 for reheating the refluxgas. The gas is conveyed through a cooler 36 and a dust separator 37,e.g. an electrofilter, before it reaches the gas compressor 34. The gaswithdrawn through the conduit 31 is preferably utilized for energyproduction in a gas turbine.

EXAMPLE

An apparatus as illustrated in FIG. 1, was built for a working pressureof 5 bar. The upper reaction chamber had a height of 20 meter and adiameter of 0.45 m. Fine-grained iron ore concentrate with an ironcontent of 67% was reduced in the following way.

Ore concentrates flowing at a rate of 300 kg/h, with a mean granularsize of 0.1 mm, was fed into the venturi preheater 17, where it washeated by gas departing from the cyclone separator 8. The preheated oreconcentrates was collected in the first dust cyclone 19 and was conveyedvia the conduit 23 down to the lower reaction chamber 3, where it wasmixed into a circulating fluidized bed. This fluidized bed wasmaintained by the reducing gas supplied to the bottom via pipes 10, andextended up into the upper reaction chamber 2. Powdered steam coalhaving an average particle size of 0.2 mm was taken in through thesupply pipe 7. This powdered coal was transformed into coke in thefluidized bed. It was supplied in such a quantity that (1) it producedthe necessary reducing gas for the reduction, (2) it kept the cokecontent in the bed between 20 and 50 percent by weight, in spite of thetapping-off of reduced material which took place, and (3) it provided,by partial combustion, the heat required for the reduction and the heatlosses from the apparatus. If the heat losses are overlooked, these notcorresponding to the losses on an industrial scale, due to the smalldimensions of the apparatus, the powdered coal supply corresponded to700 kg coal per ton Fe. Preheated air was supplied through the airnozzles 6 in the amount required for giving the necessary heat for thereduction and maintaining the temperature at 970° C. by partialcombustion of gases and carbonaceous material. By supplying reducing gasto the lower reaction chamber, the same gas velocity was set there as inthe upper reaction chamber, and the whole reactor was filled with thecirculating fluidized bed. About 5 ton/h of the solid material migratedover into the cyclone separator 8, where it was separated from the gasand recycled to the bottom of the lower reaction chamber. The gas ratein the pipe 15, connected to the discharge shaft 11 from the lowerreaction chamber, was adjusted so that a wind separation was achieved inthe shaft 11, to the effect that the lighter carbon particles were blownaway from the heavier iron particles. A product with a degree ofmetallization of 70% and containing 10% by weight of carbon wasdischarged in a quantity corresponding to the supplied quantity of ironore concentrates. The gas departing from the dust cyclone separator 19was further cleaned in the dust cyclone separator 20, the dust beingrecycled to the lower reaction chamber 3. A portion of the gas departingfrom the dust cyclone separator was cleaned in a washing means 33 fromwater and carbon dioxide, and subsequently used as fluidizing gas in thelower reaction chamber 3 and in the discharge shaft 11. The remainder ofthe gas was used for other purposes, e.g. the production of steam in anexhaust gas steam boiler and the production of electrical energytherefrom.

The wind-separation in the discharge shaft 11 may be carried so far thatthe discharged material contains practically no carbon. If desired,carbon may be added to said discharged material, e.g. dust rich incarbon taken from the dust cyclone separator 19.

We claim:
 1. Apparatus for reducing finely divided iron oxide materialin a circulating fluidized bed, comprising a reactor (1) containing avertical upper reaction chamber (2), downwardly connected to a lower,narrower, vertical reaction chamber (3), a cyclone separator (8)connected to the upper reaction chamber for separating the solidmaterial from the circulating fluidized bed, a recycling conduit (9)from the cyclone separator to the reactor, nozzles (6) for supplyingcombustion air to the reactor, means (7) for supplying a reduction agentand an iron oxide material (18), means (11) for tapping off reducedmaterial, and means (10) for supplying fluidizing gas to the lowerreaction chamber, said recycling conduit (9) for the solid materialopening into the bottom of the lower reaction chamber, said nozzles (6)for combustion air extending interiorly of an interior wall of thereactor at a distance which is at least equal to an inner diameter ofthe nozzles, and said means for tapping-off comprising a shaft (11)connected to the bottom of the lower reaction chamber (3), and of alesser cross-sectional area than the lower reaction chamber and aboutless than 1/4 of the cross-sectional area thereof, said tapping-offmeans including a sluice (13) connected to said shaft for tapping-offthe reduced material.
 2. The apparatus as defined in claim 1 wherein thesupply means for the iron oxide material consists of a first venturipreheater (17) connected to the gas outlet of the cyclone separator (8),and connected to a first dust cyclone separator (19) for separatingpreheated ore concentrates, said first dust cyclone separator beingconnected to the reactor (1) via a conduit (23), and a second venturipreheater (25) having an inlet (18) for ore concentrates and beingconnected to a second dust cyclone separator (26) for separatingpreheated ore concentrates, said second dust cyclone separator beingarranged to deliver the collected preheated ore concentrates to thefirst venturi preheater.
 3. The apparatus as defined in claim 2 andwherein the cross-sectional area of the lower reaction chamber (3) isabout 1/3-1/2 of the cross-sectional area of the upper reaction chamber(2).
 4. The apparatus as defined in claim 2 and wherein the conduit (23)from the first dust cyclone separator (19) is connected to the lowerreaction chamber (3).
 5. The apparatus as defined in claim 2 and whereinthe tapping-off means includes a fluidizing gas supply pipe (15), saidtapping-off shaft (11) and said fluidizing gas supply pipe (15) beingdimensioned and disposed such that the tapped-off reduced material isfluid flow-separated to a lower carbon content.